The most efficient insulation used in buildings and appliances is closed cell foam polyurethane, polyisocyanerate or phenolic insulation. When new, this insulation has over twice the insulating value, per inch of thickness, as does fiberglass insulations. All refrigerators made today in the United States use closed cell foam for all of the surfaces, save the doors. About seven million refrigerators are sold in the United States every year. One-third of the interior volume of a refrigerator is foam insulation; industry representatives estimate that reducing insulation volume by one cubic foot would be worth $50.
About one-half of the buildings constructed used closed cell foam for roof or wall insulation. Use of boards made of foam as exterior sheathing is the most practical way 2.times.4 framed construction can meet the current energy standards.
Closed cell foam has excellent insulating characteristics because it contains chlorofluorocarbon, CFC, vapor within the cells. The CFC vapor has a thermal conductivity which is one-third that of air. Recently, it has been found that this CFC causes depletion of the stratospheric ozone layer. The United States and other developed nations have agreed to phase out the CFC use over the next decade. Replacement refrigerant vapors are not available at present. Replacements being considered have not completed toxicity tests, have higher projected costs, and there is little manufacturing capability in place for these replacements.
The replacements blowing agents also have higher thermal conductivity than present blowing agents. For the next decade, as CFC usage is phased out, manufacturers will probably use a foam which is blown with a mixture of CFC and CO.sub.2. This mixture will yield a foam with a higher thermal conductivity than pure CFC blown foam.
To maintain present levels of energy efficiency, or appliances, there is a need to develop techniques which will reduce the overall thermal conductivity of foams blown with replacement blowing agents. In a foam filled with CFC, one-half of the heat transfer is by conduction through the vapor in the cell interior, one-third is by thermal radiation in the infrared wavelengths, and the balance is by conduction through the structure of the solid polymer. The radiation heat transfer is substantial because the cell walls are transparent to radiation and only the cell struts, formed at the intersection of three cell walls, are opaque.
If the cell walls of a foam can be made opaque, the radiation heat transfer can be reduced by more than 50%. Several investigators have tried adding opaque powders, or flakes, to the foam. In the Schuetz, M.S. thesis, M.I.T. (1982), experiments in the use of an opaque coal dust is described. More recently, in Torpey's thesis, M.I.T. (1987), the use of graphite and aluminum flakes is described. They showed that the overall thermal conductivity could be reduced by between 5 and 10% by the use of these materials. However, Torpey also shows that as a larger proportion of the flakes were added, the foam became more opaque, but the thermal conductivity reached a minimum and then began to rise as still more flakes are added to the polymer. U.S. Pat. No. 4,795,763, issued Jan. 3, 1989, describes a closed cell, rigid, polymer foam containing an opaque carbon black filler to reduce the aged K-factor of the foam.